1. FIELD OF THE INVENTION
The present invention relates to a power supply system in which a plurality of AC output inverting devices like inverters are connected and operated in parallel with respect to common load, and more particularly to a means for controlling the current balance between inverting devices for use in the system.
2. DESCRIPTION OF THE RELATED ART
FIG. 10 shows a schematic view of a parallel operational system of a conventional AC output inverter disclosed in, for example, Japanese Patent Publication Nos. 53-36137 and 56-13101.
Referring to FIG. 10 , a first inverter device 11 operates in parallel with a second inverter device 12, which has like construction, through an output bus 13 and supplies electric power to a load 14. The first inverter device 11 is mainly composed of an inverter body 110, a reactor 111 and a condenser 112 serving as a filter. The first inverter device 11 power, and is connected to the output bus 13 through an output switch 113a. In order to operate the first and second inverter devices 11 and 12 in parallel, a detection signal I.sub.1a is obtained from an output current I.sub.1 of the first inverter device 11 by a current transformer (CT) 120a, and a difference between the detection signal I.sub.1a and a detection signal I.sub.2a similarly obtained from the second inverter device 12, that is, a signal .DELTA.I.sub.1 corresponding to cross current is obtained by a cross current detector 151. Then, two orthogonal voltage vectors E.sub.A and E.sub.B are generated by a phase shifter 150, and a reactive power corresponding component .DELTA.Q and an active power corresponding component .DELTA.P are obtained from the signal .DELTA.I.sub.1 by arithmetic circuits 152 and 153, respectively. A voltage control circuit 143 performs pulse width modulation for the inverter body 110 through a PWM circuit 140 based on signals from a voltage setting circuit 17 and a voltage feedback circuit 130, thereby controlling the internal voltage.
The above reactive power corresponding component .DELTA.Q is supplied as a supplementary signal to the voltage control circuit 143, and the internal voltage of the inverter body 110 is adjusted by at most several percent sc that .DELTA.Q becomes 0.
On the other hand, the active power corresponding component .DELTA.P is input to a reference oscillator 155 through an amplifier 154 as a component of a PLL circuit, and the phase of the internal voltage of the inverter body 110 is controlled by finely adjusting the frequency of the reference oscillator 155 so that .DELTA.P becomes 0.
Since the voltage and the phase are thus controlled so that both .DELTA.Q and .DELTA.P become 0, no cross current exists between the two inverters and stable load sharing is achieved.
However, the conventional parallel operational system of inverters has the following three problems. First, since shared currents are balanced by controlling the phase and an average value of the internal voltages of the inverters, it is difficult to improve the response speed of control, and, in particular, it is impossible to control instantaneous cross current. Secondly, since a filter is necessary to detect an active component and a reactive component of the cross current separately, the cross current cannot be controlled at high speed. Therefore, there is a limit in applying the system to high speed voltage control, for example, instantaneous waveform control which ensures that an output of the inverter a sine wave of high quality with little distortion. Thirdly, since the active component and the reactive component of the cross current are separately controlled, the control circuit is complicated.